Increased current for the bridge sensor with PGA900 and 3-wire transmitter in 4..20mA current mode

Antonio Sierra

Other Parts Discussed in Thread: PGA900, OPA551, XTR111

Hi,

We want to design a 3-wire transmitter, but the supply current for our sensor needs to be higher than the maximum PGA900 excitation current. The inputs to our circuit are the following ones:

- Positive supply (+12V DC)

- Negative supply (GND for the positive supply)

- Signal output (4..20mA)

We have tried to follow the TI’s reference circuit but our problem is that our bridge sensor is a catalytic gas bead and it needs about 180mA of excitacion current. TI’s design is made for 2-wire transmitters and ir our case, we could work with a 3-wire connection because we have a direct connection to the supply GND. We will like to separete the 4..20mA signal from the supply line return path.

Please, see attached file with a circuit proposal. We are not sure about the current return paths. Where should we connect the return path (GND) for the SMPS converter and for the VBRGN terminal?

We will appreciate any help you can give us. Thank you very much.

over 9 years ago

0 Matthew Sullivan over 9 years ago

TI__Intellectual 2905 points

Antonio,

The best solution that I can think for your requirements is to use a buffer capable of driving 180mA, something like the OPA551, and using that to drive the bridge.This would be better than driving the bridge using a SMPS, as it allows you to still use the PGA900's accurate, low drift bridge excitation, but a buffer to get the current you need.

Then, for your 3-wire 4 to 20mA solution, you can use a circuit similar to the Iout configuration found in this blog post: http://e2e.ti.com/blogs_/b/precisionhub/archive/2014/05/23/industrial-dacs-an-evolution-of-3-wire-analog-outputs

The DAC/VOUT stage of the DAC862 is replaced with the internal DAC/Output stage of the PGA900.

0 Hawk Tong over 9 years ago in reply to Matthew Sullivan

TI__Intellectual 1840 points

I don't think your concept figure here , is not essentially one 3 wires current loop solution. In my understanding, PGA900 voltage output mode + XTR111 may be suitable combination for your case.
Hawk Tong

Sensors

Sensors forum

Increased current for the bridge sensor with PGA900 and 3-wire transmitter in 4..20mA current mode